The present invention relates generally to infrared radiation (IR) sources for general application in instrumentation systems including clinical laboratory, industrial, and field instrumentation systems; more particularly, the invention relates to a novel lamp which overcomes the problems associated with prior art IR sources. In addition to analytical, imaging and diagnostic instrumentation, such IR sources have been used in identification of targets in military operations and more particularly for optical "Identification--Friend or Foe" (IFF) systems in which a potential target is interrogated by a signal from an attack aircraft and, if no coded signal is returned, the target is deemed to be legitimate and can be attached. Such systems using interrogation and response signals in the UHF bands have long been used and have used various secrecy coding methods, including pulse code modulation. Later IFF systems utilize infrared wavelengths.
Prior infrared devices for analysis instrumentation involve several limitations, such as short life, low color temperature, fragile construction, high power consumption, starting difficulties, and restricted wavelength emission.
There have been basically four types of infrared sources in use: Nernst glower, globar, gas mantle, and tungsten filament lamps.
The Nernst glower is a device made of a refractory material such as thoria. It is heated by a flame to start, then a controlled current is passed through to maintain and control the desired temperature. The glower typically operates at 200 watts, 60 amperes, and at 1500 to 1950 degrees K temperature. It is continuously energized to insure that it will not break when cooled. The life of the glower depends on operating temperature and careful handling. Lifetimes of 200 to 1000 hours are claimed by various manufacturers.
The gas mantle utilizes a gasoline fired mantle similar to a camping lamp. The mantle is made of thoria and when heated produces a strong emission. The main problems are the hazards of the gaseous fuel, and that its output spectrum does not approximate an ideal black-body curve.
Tungsten lamps have been used extensively in the visible and near-infrared for many years because of the broad choice of packaging and power levels. Tungsten is an ideal choice of filament material, especially when used in the "Halogen Cycle" to produce higher color temperatures. The disadvantage is that quartz is the only choice for envelope material, limiting the spectral emissions to the region of 3 microns maximum.
The globar is a rod of silicon carbide, which is electrically heated to the desired temperature. It does not require auxiliary heat to start, but requires a flow of water to cool the housing. Typical power is 200 watts, 6 amperes, with color temperatures of 1470 degrees K. Silicon carbide infrared radiation sources are commonly used in the prior art as broad band infrared radiation sources in many infrared spectroscopic instruments. One of the difficulties involved in their extensive use is that they have a failure mode wherein they often crack and become open circuited if powered to high color temperatures over 1500 degrees Kelvin. Environmental factors such as gaseous contamination, thermal stress, and oxidation at the high operating temperature of these devices, as well as vibration and mounting problems, contribute to the possibility of failure of these infrared sources.
It is therefore desirable to provide a closed environment and a reliable mounting for the radiation source material. These systems must consider chemical composition and physical properties such as expansion and contraction over the temperature range.
A closed environment is obtainable by mounting the infrared radiation source in a closed envelope filled with an inert gas free of oxygen and contaminants which would react with the material of the radiation source. Expansion and contraction of the source can be accommodated by filament mounting techniques, e.g., coiled or looped. The source can be further improved by integration with elements to achieve specific optical characteristics, such as output wavelength, collimation, focal length, directionality, and integration with external elements of an optical system.
This lamp makes possible process control instruments for remote locations and portable applications due to its small size, low power consumption, long service life, easy starting, and high color temperature and spectral emission.